Transistor gate drivers are used to set and control gate voltages applied to the gates of transistors. In some scenarios, it may be desired to maintain a particular source voltage, for example to ensure that the source terminal of a field-effect transistor is at a suitable voltage to drive a load that is coupled to the source terminal. The source voltage will be approximately equal to the applied gate voltage, less the gate-source voltage of the transistor (VGS). Therefore, in order to keep a transistor turned on with a desired source voltage (VS) at the source terminal of the transistor, transistor gate drivers generally set the gate voltage to equal VS+VGS.
One factor that determines the VGS of a transistor is the turn-on threshold voltage (VTH). Not only does this vary between different types and designs of transistor, but it also varies between individual transistors of the same type and design as a result of manufacturing tolerances (typically, manufacturers may quote a particular VTH for a transistor, with a tolerance of ±1V). Another factor that determines the VGS of a transistor is the operating state of the transistor. A major aspect of the operating state is the channel current through the transistor (IDS). It is a characteristic of transistors that the magnitude of VGS increases as IDS increases (for n-type transistors, VGS is a positive value that will increase as IDS increases and for p-type transistors VGS is a negative value that will become more negative as IDS increases). As a result, for an n-type transistor, as IDS increases, the transistor gate driver will typically increase the gate voltage in line with the increase in VGS, thereby maintaining the desired source voltage. Conversely, for a p-type transistor, the transistor gate driver will typically make the gate voltage more negative in line with the more negative VGS, thereby maintaining the desired source voltage.